Calcium nitrate has become a large and important product in the fertilizer field and also for many other technical applications such as waste water treatment, the concrete industry, etc.
Solid crystalline calcium nitrate is commonly found as hydrates such as Ca(NO3)2.4H2O, Ca(NO3)2.3H2O and Ca(NO3)2.2H2O. Ca(NO3)2.4H2O is a very common salt with a melting point of 42° C. It contains 68-69 weight % of calcium nitrate and about 30 weight % of water.
The traditional method for making calcium nitrate tetra hydrate [Ca(NO3)2.4H2O] is by means of crystallization of a solution high in calcium nitrate which is cooled down below the saturation point of the calcium nitrate tetra hydrate. The solution is then filtered through which wet calcium nitrate tetra hydrate crystals are deposited on a filter. The crystals are removed from the filter and dried under vacuum at a temperature below 40° C.
Prilled or granulated calcium nitrate is commercially available. The normal composition of this material is between 77 and 80 weight % of Ca(NO3)2, between 6 and 8 weight % of ammonium nitrate (AN) and between 14 and 17 weight % of crystallization water. Therein, the salt AN*5CN*10H2O, abbreviated as 1:5:10, constitutes a major part.
Prilled or granulated calcium nitrate products are produced by forming particles (e.g. prills, granules) of a melt of calcium nitrate, ammonium nitrate and water. Typically, such a melt has a water content of between 15 and 18 weight % of water and between 5 and 8 weight % of ammonium nitrate in addition to the calcium nitrate. The ammonium nitrate and the water are necessary to form a melt around 100° C. and 110° C. The ammonium nitrate content is necessary to make the melt solidify quickly. If the ammonium nitrate is removed, the solidification process proceeds so slowly that normal prilling or granulation methods cannot be applied.
In WO 2004/039722, a method is described for the production of nitrate containing products from undercooling melts. More specifically, a cooling belt is used for solidification of the particles. This method is amongst others used to produce calcium nitrate in the form of prills, granules or pastilles. This procedure is not applicable to make pure calcium nitrate without crystal water since the melting point of pure anhydrous calcium nitrate is more than 500° C.
For some applications however, there exists the need for pure, anhydrous calcium nitrate, i.e. calcium nitrate which is free of ammonium nitrate or any other salts and free of crystal water.
In GB 392,531, a process is disclosed for the preparation of non-caking fertilizers containing calcium nitrate, in which calcium nitrate solutions are evaporated to a sandy-pasty consistency in which they contain about 90-95% of calcium nitrate (calculated as anhydrous calcium nitrate), and then converting the concentrated product without any substantial further concentration into granular form by mechanical disaggregation in bulk at a temperature substantially above atmospheric of the order of 50 to 100° C. In example 1, an amount of calcium nitrate tetra hydrate is melted in a kneading machine adapted to be heated, where after this melt is further evaporated under vacuum to a content of calcium nitrate of 94%, while the temperature is raised to 90° C. The melt, at first thinly liquid, becomes rapidly thickened and immediately afterwards changes over to a sandy-pasty mass. This mass is then cooled to 80° C. and can easily be brought into a granular form by mechanical disaggregation at 80° C.
The disadvantage of the method as described in GB 392,531 however is that it gives rise to salt deposits on equipment and a flaked, dusty product. Therefore, this method is not well suited for large-scale production of calcium nitrate.
CS 151689 B1 relates to production of powdered calcium nitrate solutions dried with hot air, the solution having a concentration of 15 to 70 weight % calcium nitrate dried in a stream of hot air with an inlet temperature of 130-500° C. and an outlet temperature of at least 80° C.
WO 2007/012951 A1 relates to a method of producing anhydrous calcium nitrate, anhydrous magnesium nitrate or a mixture thereof, which includes providing a solution of calcium nitrate, magnesium nitrate or a mixture thereof, and removing water therefrom in a batch working pulse combustion drier.
A disadvantage of the methods from CS 151689 B1 and WO 2007/012951 A1 is that the calcium nitrate powder produced can be too wet/big or too dry/dusty.
The aim of the present invention is to form an anhydrous calcium nitrate having high to very high CN concentrations (also denominated as “pure” anhydrous calcium nitrate), that can be mixed with other powdery systems, therewith avoiding segregation. A further aim of the present invention is to form an anhydrous calcium nitrate powder with a determined target particle size between 0.05 and 1.5 mm, preferred water content between 0.1 and 8 weight % and calcium nitrate content between 92 and 99.9 weight %. A further aim of the present invention is to form a calcium nitrate powder wherein the moister content is within a value suitable for handling. A further aim of the present invention is provide a method for producing calcium nitrate powder wherein clogging of particles during the process is avoided. A further aim of the present invention is to provide such a highly concentrated anhydrous calcium nitrate that can dissolve quickly.